| Rare earth (RE) ion doped phosphors have attracted great interest during the past several decades due to their unique physical and chemical properties. RE ions can display many significative properties in optics, electronics, and magnetics, originating from f-f electronic transitions within the 4f shell. Among these RE ions, the Eu3+ ion is an important activator that can emit red fluorescence centered at around 616 nm, corresponding to the 5Do-F2 transition, while located in a noncentrosymmetric site. Thus, many materials doped with Eu3+ can be used as red phosphors and have potential application in color television displays. It is reported that metal molybdates have a high application potential in various fields such as photoluminescence, microwave applications, optical fibers, humidity sensors, and catalysis. Lately, more and more attention is paid to red-emitting phosphor based on Eu3+ doped molybdates used for white-light-emitting diodes (LEDs) because of their intense charge transfer (CT) absorption bands in near-UV and effective f-f transition of Eu3+. It is well known that white LEDs can offer benefits in terms of high luminous efficiency, maintenance, and environmental protection. In order to obtain a higher efficiency white LED with an appropriate color temperature and a higher color-rendering index, a new approach using near-ultraviolet (nUV) InGaN-based LED chip coated with blue/green/red tricolor phosphors was introduced. However, the lack of effective red phosphor blocks the development of white LEDs because few red phosphors could be excited efficiently by blue or nUV light. Therefore, it is high interest to search for a suitable red phosphor for fabrication of LEDs. Recently, the template-free hydrothermal method has been considered as a good synthesis process for some inorganic powders because of available synthesis of crystallized products at low reaction temperature, flexibility in the design of reaction conditions, uniformity of product composition, phase, and microstructure, and simplicity of equipment and processing. In this paper, uniform and well-crystallized octahedral NaGd(MoO4)2:Eu3+ microcrystals have been successfully synthesized by a facile one-step hydrothermal synthesis method without involving any templates. The prepared samples were systematically characterized by powder X-ray diffraction (XRD), field emission-scanning electron microscopy (FE-SEM), photoluminescence (PL), and potoluminescent excitation spectra (PLE). It was found that the starting pH value played an important role in the pure-phase formation and uniform morphology of octahedral microcrystals. Detailed proofs indicated that the growth process of NaGd(MoO4)2:Eu3+ microcrystals was dominated by a nucleation-crystallization-oriented attachment mechanism. Furthermore, the luminescent properties of the as-synthesized NaGd(MoO4)2:Eu3+ microcrystals were investigated, demonstrating that the PL intensity was influenced by the different morphologies, and the bipyramidal octahedra NaGd(MoO4)2:Eu3+ luminescent microcrystals might be applied as an excellent red component for near-ultraviolet white light emitting diodes (LEDs).Much attention was paid to the molybdates doped with Eu3+, nevertheless, most of the previous works were mainly concentrated on the material synthesis and luminescence, and little has been done on the luminescence of the europium dependence of the molybdate crystal structure. Eu3+ luminescence is of special importance as a spectral probe from its application in phosphor materials. This is possible for the reason that Eu3+ has several structure-dependent transitions enabling one to gain insight about the site that it occupies in a given host. On this basis we have attempted to explore the different cationic sites present in the molybdates system. In this work, the site-selective excitation and emission spectra along with the decay times have been investigated under the pulsed dye-laser excitation in the 5Do-Fo region of Eu3+ in R2(MoO4)3 (R=La, Gd and Y). We observed clearly two crystallographic sites for Eu3+ in La2(MoO4)3 and one crystallographic site for Eu3+ in Gd2(MoO4)3, however, Eu3+ occupies more than one crystallographic site in the Y(MoO4)3, because different sites may possibly are too closed to one site or couldn't be observed in the room temperature. The stark energy levels for Eu3+ at different site were assigned from site-selective emission spectra. The luminescent properties and morphologies of the as-synthesized samples were also investigated, the La2(MoO4)3:Eu3+ and Gd2(MoO4)3:Eu3+ phosphors which showed strong line at 395 nm in excitation spectra, intensive red emission and suitable morphology for LED, indicating that they could be used as red components for white light-emitting diodes. The photoluminescence decay curves of the as-obtained phosphors were also investigated. The results show that the lifetime is short enough for potential applications in displays and lights. |
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